Prime Minister Scott Morrison has urged voters in the Wentworth by-election to ignore the son of his ousted predecessor Malcolm Turnbull, who told them to abandon the Liberal Party because it has been taken over by "extremists".

Key points:

Alex Turnbull said the Wentworth by-election was a chance for voters to "send a signal" to the Liberal Party

Alex Turnbull took to social media to issue the blunt assessment, saying the party's "hard right" had little motivation to win the election or serve the public, and wanted only to "pursue a crazy agenda".

Mr Morrison said Alex Turnbull was "his own person", but that his father Malcolm Turnbull was heavily supporting the Liberal candidate Dave Sharma.

"At the Wentworth by-election, I have a simple message, if you want a continuation of the certainty for our economy and of economic management, voting for anyone other than Dave Sharma puts all of that at risk," Mr Morrison said.

"If an independent is elected at the Wentworth by-election, that will throw us into a hung parliament and a lot of uncertainty at a time when the country doesn't need it."

Asked about Malcolm Turnbull's "noticeable silence" since initially backing Mr Sharma, Mr Morrison said the former PM had left Australian public life and politics.

In his video, Alex Turnbull, who runs a hedge fund in Singapore, suggested the poll should be seen by voters as "an intervention".

"We're going to have an election in 12 months anyway, so if you want to send a signal as to which way the Liberal Party is going, and your displeasure with where it's going, then this is your opportunity," he said.

"Don't vote for the Liberal Party in the Wentworth by-election.

"If you want to pull the Liberal Party back from the brink, it's the one clear signal you can send."

He said the best way for voters to tell the Liberals they were unhappy with constant leadership change was to vote them out.

"To me, this particular event seems to show the Liberal party has been taken over, frankly, by extremists on the hard right who aren't particularly motivated to win an election and aren't particularly motivated to serve the general public.